1. Field of the Invention
The present invention relates to a method of forming an active matrix organic light emitting display (AM-OLED) and, more particularly, to a method of forming an amorphous-silicon thin film transistor (a-Si:H TFT) in an AM-OLED.
2. Description of the Related Art
In accordance with driving methods, organic light emitting displays (OLED) are classified into active and positive matrix types. The active matrix organic light emitting display (AM-OLED) is driven by electric currents, in which each of the matrix-array pixel areas has at least one thin film transistor (TFT), serving as a switch, to modulate the driving current based on the variation of capacitor storage potential so as to control the brightness and gray level of the pixel areas. At present, the AM-OLED is driven by two TFTs in each pixel area, and alternatively the AM-OLED is driven by four TFTs in each pixel area.
Concerned with the fabrication of the TFTs, a polysilicon TFT has advantages of high mobility, great integration of driving circuits, small leakage current and good applications for high-speed devices, thus the current TFT process for the AM-OLED application has a tendency toward a low-temperature polysilicon (LTPS). However, the LTPS requires a lengthy photolithography-etching process, including as many as eight or nine photolithography-etching steps, resulting in high process costs, degraded yield and a difficulty in mass production of large-size panel applications. Comparatively, an amorphous silicon (a-Si:H) TFT process with superior process costs and product properties, with as few as five or six photolithography-etching steps, has been popularly applied to the fabrication of large-size liquid crystal displays (LCDs) Accordingly, integration of the a-Si:H TFT process into the AM-OLED process is currently an important issue.